In advertisements on straps provided in cars of public transportation, such as trains or buses, some advertisements, which have conventionally been displayed using paper media, have recently been displayed using flat-panel displays, such as liquid crystal panels or electro-luminescence (EL) displays. In such strap signage systems, there is a demand for more flexible display; for example, it is required to display different advertisements in each car.
When different advertisements are displayed on straps in each car, it is required to transmit information (car numbers) on the positions of cars to the straps. The position information may be transmitted by visible light communication that transmits information by superimposing the information on illumination light.
In the visible light communication using illumination light, each of illumination devices mounted on ceilings of cars transmits information on the position of the car on which the illumination device is mounted, by superimposing the information on light of the illumination device. The straps obtain the position information from the illumination devices mounted on the ceilings and display advertisements corresponding to the position information.
One advantage of visible light communication is that since visible light has directivity, interference is less likely to occur as compared with wireless communication. Thus, no interference occurs among the cars, which makes it possible to obtain proper position information.
Standards for visible light communication using illumination light include JEITA CP-1223. CP-1223 transmits information using a modulation method called PPM. PPM is a method that transmits information by mapping the information to the position of a pulse with a given width; in M-PPM, a symbol is divided into M slots, a pulse is placed at a position of a slot corresponding to information to be transmitted. FIG. 12 illustrates an example of 4-PPM when M=4. By turning off illumination light when a PPM signal is 1 and turning on the illumination light when the PPM signal is 0, it is possible to transmit information using the illumination light.
Since PPM keeps the ratio of 0 and 1 constant, it has an advantage that no flicker appears in illumination light on which information is superimposed.
CP-1223 is mainly used for transmission of data, such as ID or position information, having a small amount of information. Thus, it does not constantly transmit information, and a transmitted signal includes an invalid signal having no information.
A transmitted signal has a configuration in which there is an invalid signal for an indefinite period of time between frames for transmitting information (see FIG. 13); a frame consists of a known signal and 76 4-PPM symbols that are a data signal. The known signal has a unique sequence “111000000000” that does not appear in any PPM symbols and any invalid signals.
PPM maps information to the distance (time) from the head (symbol header) of a symbol to the position at which a pulse exists. For example, in 4-PPM, when information “00” is transmitted, a pulse is placed in the slot immediately after the symbol header of a symbol. When information “11” is transmitted, a pulse is placed in the slot farthest from the symbol header of a symbol. Thus, in demodulation for extracting information ‘0’ or ‘1’ from a symbol, it is required to accurately reproduce the position of the symbol header.
When a frame is transmitted, the transmission side first superimposes the known signal on illumination light and then superimposes 76 4-PPM symbols on the illumination light. At this time, a symbol time, which is a time interval at which 4-PPM symbols are transmitted, is measured on the basis of a clock.
Next, processing on a receiving side will be described. On the receiving side, a photodetector first converts the intensity of the illumination light into a voltage. Thereby, ON/OFF information of the illumination light is obtained as an electrical signal. Then, an A/D converter converts the voltage into discrete digital data (digital sampled values) on the basis of a sampling clock.
Further, a reference position detector detects the known signal in the sampled values sent from the A/D converter, and calculates, on the basis of the position of the detected known signal, the positions of the 76 symbol headers following the known signal.
Specifically, the position of the leading symbol header is determined from the position of the known signal, and the positions of the symbol headers existing at intervals of the symbol time are obtained on the basis of the determined position of the symbol header. At this time, since the number of samples per symbol time is equal to Ts×fs (where Ts is the symbol time, and fs is the frequency of the sampling clock), the position of the sample located Ts×fs samples ahead of the sample at the position of a symbol header is determined as the position of the next symbol header. Thus, the symbol time depends on the sampling clock.
However, in general, a clock of a product includes an error due to performance variations of components caused in the manufacturing process. Thus, the transmitting side symbol time measurement clock for measuring the symbol time on the transmitting side and the sampling clock for measuring the symbol time on the receiving side include errors. This causes a difference between the symbol time on the transmitting side and the symbol time on the receiving side.
Regarding a clock error that is a deviation of the clock on the transmitting side or the clock on the receiving side from a designed value, the transmitting side symbol time measurement clock, which is the clock on the transmitting side, and the sampling clock, which is the clock on the receiving side, will be considered.
FIG. 14 is a schematic diagram illustrating deviations of the positions of symbol headers due to a clock error in a prior art. As can be clearly seen from the diagram, the symbol time on the transmitting side derived from the transmitting side symbol time measurement clock and the symbol time on the receiving side derived from the sampling clock do not completely coincide with each other. Thus, it is difficult to make the positions of the symbol headers of 4-PPM symbols transmitted from the transmitting side and the positions of the symbol headers calculated on the receiving side accurately coincide with each other, and the deviation gradually increases.
The tolerance of data transmission rate in CP-1223 is permitted up to 0.5%, and it is considered that there is a clock error of up to ±0.5% on the transmitting side. Thus, a receiving device conforming to CP-1223 needs to detect and correct a clock error of ±0.5%.
As one of methods of detecting and correcting a clock error, there is a method of inserting a pilot signal for correcting a clock error into data to achieve accurate synchronization; however, there is a problem in that transmission efficiency decreases as the inserted pilot signal increases.
To solve the problem described above, as a conventional method, as illustrated in FIG. 15, there is a method of synchronizing symbol headers when the interval between pulses is one of given values, without inserting a pilot signal. In M-PPM, when the interval between pulses is 2×(M−1) slots, the preceding pulse is necessarily present in the first slot in a symbol, and the following pulse is necessarily present in the last slot in a symbol. Moreover, when two pulses are continuous, the preceding pulse is necessarily present in the last slot in a symbol, and the following pulse is necessarily present in the first slot in a symbol. The positions of the pulses in the symbols become clear, and thus the positions of the symbol headers are determined, so that the clock error can be corrected (see, for example, Patent Literature 1).